Line reflection reduction with energy-recovery driver

ABSTRACT

A system and method for reducing reflections in a transmission line and for recovering energy from the load in the transmission during the process. At least three drive signal levels are utilized. The transition from the second level to the third level during a rising transition and the transition from the second level to the first level during a falling transition is timed to coincide with the arrival of the reflected signal from the immediately-preceding transition. A capacitor is advantageously used as the source for the intermediate drive signal levels and advantageously facilitates energy recovery and conservation.

CROSS-REFERENCE TO RELATED APPLICATION

[0001] This application is a continuation of application Ser. No.09/532,798, filed Mar. 21, 2000.

[0002] Application Ser. No. 09/532,798 claimed the benefit of U.S.Provisional Application No. 60/125,580, filed Mar. 22, 1999, thecontents of which are incorporated herein by reference.

GOVERNMENT LICENSE RIGHTS

[0003] The invention was made with government support underDAAL01-95-K-3528 sponsored by DARPA and MDA904-93-C-L042 sponsored byDOD. The government has certain rights in the invention.

BACKGROUND OF THE INVENTION

[0004] 1. Field of the Invention

[0005] This invention relates to line drivers and, more particularly, totechniques for reducing ringing and power losses in line drivingsystems.

[0006] 2. Description of Related Art

[0007] Line reflection decreases the noise margin in high-speed digitalcircuits, especially line reflections induced by signal buffers drivingoff-chip loads.

[0008] The signal buffer acts as a line driver. After the driver causesa signal transition, the transition travels from the source (near) endof the transmission line to the load (far) end. Upon reaching the end,the signal transition is usually reflected at the far end and travelsback toward the source. The reflected signal is then usually againreflected upon reaching the source back towards the load. This processcontinues until the cumulated losses cause the reflection to die out.The resulting voltage waveform seen at either end of the transmissionline is typically an exponentially-damped oscillation, often referred toas “ringing.”

[0009] Ringing often creates problems. It often causes the voltage onthe transmission line to exceed allowable or safe levels. Therefore,circuitry connected to the line must be designed to accommodate highervoltage levels than are actually needed. The ringing can also beerroneously interpreted as a change in the state of the data on theline.

[0010] In the past, efforts have been made to match the impedance of thedriver to the transmission line and/or the impedance of the transmissionline to the load. If the impedance at one end or the other is perfectlymatched, there would normally be no reflection.

[0011] A simple approach used to match the impedance between thetransmission line and the load is to connect a resistance at the end ofthe transmission line to ground. This approach, however, causesadditional power to be dissipated in the resistance that is added. Thisis undesirable in low-power applications, such as in a VLSI pin driverused for fast chip-to-chip communication.

[0012] A simple approach for matching the impedance between the driverand the transmission line is to insert a resistance in series betweenthe output of the driver and the input of the transmission line. Again,however, the addition of such a resistance increases power dissipation.

[0013] Another problem with line driving systems is the dissipation ofpower that occurs during transitions of the signal. This is particularlytrue when the load includes a substantial capacitive reactance, such asin a VLSI pin driver used for fast chip-to-chip communication.

[0014] In short, there is a need for a driver that drives a transmissionline connected to a load which substantially reduces ringing withoutwasting power and which, preferably, reduces the energy that isdissipated during operation.

SUMMARY OF INVENTION

[0015] One object of the invention is to obviate these as well as otherproblems in the prior art.

[0016] Another object of the invention is to reduce ringing intransmission-line driving systems.

[0017] A still further object of the invention is to reduce ringing intransmission-line driving systems without increasing power dissipation.

[0018] A still further object of the invention is to reduce the maximumvoltage-level specification of circuitry that is connected to atransmission-line driving system.

[0019] A still further object of the invention is to reduce data errorscaused by transient signals in transmission-line driving systems.

[0020] A still further object of the invention is to conserve energyconsumed by a transmission line and the load it drives.

[0021] These as well as still further objects of the invention areachieved by an apparatus and method that transition the input signal toa transmission line in a plurality of steps and that cause at least oneof those steps to coincide with the arrival of a reflected signal backat the input of the transmission line.

[0022] In one embodiment of the invention, a signal generation systemgenerates at least a first drive signal, second drive and third drivesignal, the second drive signal having a level greater than the firstdrive signal, and the third drive signal having a level greater than thesecond drive signal. A controller is in communication with the signalgeneration system to cause the signal generation system to deliver thefirst drive signal, then second drive signal, and then third drivesignal to the input of the transmission line. The third drive signalbegins to be delivered to the input of the transmission lineapproximately when a reflection of the second drive signal from theoutput of the transmission line first arrives at the input to thetransmission line.

[0023] In a still further embodiment of the invention, the controlleralso causes the signal generation system to deliver the third drivesignal, then second drive signal, and then drive first drive signal tothe input of the transmission line. During this phase, the first drivesignal begins to be delivered to the input of the transmission line atapproximately when a reflection of the second drive signal from theoutput of the transmission line first arrives at the input to thetransmission line. In this embodiment, the signal generation systempreferably includes a source of the second drive signal that includes anenergy storage device, such as a capacitor. In a preferred embodiment,the capacitor receives all of its charge solely from the transmissionline.

[0024] In a still further embodiment of the invention, the level of thesecond drive signal is approximately midway between the level of thefirst drive signal and the third drive signal.

[0025] In a still further embodiment of the invention, the level of thesecond drive signal is approximately equal to the reflected level of thefirst drive signal and the reflected level of the third drive signal.

[0026] In a still further embodiment of the invention, the signalgeneration system also generates a plurality of drive signals, inaddition to the first drive signal, second drive signal and third drivesignal. In a preferred embodiment, the source for each of the pluralityof additional drive signals includes an energy storage device, such as acapacitor. Preferably, each capacitor receives all of its charge solelyfrom the transmission line.

[0027] In a still further embodiment of the invention, the signalgeneration system includes a supply for generating each of the drivesignals and a switching system that selectively connects each of thedrive signals to the input of the transmission line. In this embodiment,the controller controls the switching system.

[0028] The invention also includes a process for driving a transmissionline connected to a load, the transmission line having an input and anoutput.

[0029] In one embodiment of the process, a first drive signal, seconddrive signal and third drive signal is generated. The second drivesignal has a level greater than the first drive signal; and the thirddrive signal has a level greater than the second drive signal. The firstdrive signal, second drive signal and then third drive signal isdelivered to the input of the transmission line. The delivery of thethird drive signal begins approximately when the reflection of thesecond drive signal from the output of the transmission line firstarrives at the input to the transmission line.

[0030] In another embodiment of the process, the third drive signal,second drive signal and then first drive signal is also delivered to theinput of the transmission line. In this embodiment, the first drivesignal begins to be delivered to the input of the transmission line atapproximately when a reflection of the second drive signal from theoutput of the transmission line first arrives at the input of thetransmission line.

[0031] In a still further embodiment of the process, a source is used toprovide the second drive signal and includes an energy storage device,such as a capacitor. Preferably, the capacitor receives all of itscharge solely from the transmission line.

[0032] In a still further embodiment of the process, the level of thesecond drive signal is approximately midway between the level of thefirst drive signal and the third drive signal.

[0033] In a still further embodiment of the process, the level of thesecond drive signal is somewhat above the midway level on the risingtransition and somewhat below the midway level on the fallingtransition.

[0034] In a still further embodiment of the process, a plurality ofdrive signals are generated, in addition to the first drive signal,second drive signal and third drive signal. Preferably, a source is usedto generate each of the additional drive signals, each source includingan energy storage device, such as a capacitor. Preferably, eachcapacitor receives all of its charge solely from the transmission line.

[0035] In a still further embodiment of the process, a supply generateseach of the drive signals and a switching system selectively connectseach of the drive signals to the input of the transmission line. In thisembodiment, a controller controls the switching system.

[0036] In a still further embodiment of the invention, a driver drives atransmission line having an input connected to the driver and an outputconnected to a capacitive load. The driver includes a high-potentialvoltage source; a first electronically-controlled switch connectedbetween the high-potential voltage source and the input of thetransmission line; a low-potential voltage source; a secondelectronically-controlled switch connected between the low-potentialvoltage source and the input of the transmission line; an energy storagedevice, such as a capacitor; a third electronically-controlled switchconnected between the capacitor and the input of the transmission line;and a controller connected to the first, second and thirdelectronically-controlled switches. The controller causes the thirdelectronically-controlled switch to close approximately when a signalinjected into the transmission line by the secondelectronically-controlled switch returns back to input of thetransmission line. The controller also causes the firstelectronically-controlled switch to close approximately when a signalinjected into the transmission line by the secondelectronically-controlled switch returns back to the input of thetransmission line.

[0037] These as well as still further objects, features and benefits ofthe invention will now become clear from a review of the followingdetailed description of the preferred embodiments, read in conjunctionwith the accompanying drawings.

BRIEF DESCRIPTION OF DRAWINGS

[0038]FIG. 1 illustrates a prior art line driver system.

[0039]FIG. 2 illustrates the ringing that typically occurs in the priorart line driver system shown in FIG. 1.

[0040]FIG. 3 illustrates a line driver system made in accordance withone embodiment of the invention.

[0041]FIG. 4 illustrates the reduction in the ringing that typicallyoccurs with the embodiment of the invention shown in FIG. 3.

[0042] FIGS. 5(a) and (b) illustrate two other input wave shapes thatare useful in other embodiments of the invention.

[0043]FIG. 6 illustrates a line driver system made in accordance withanother embodiment of the invention that additionally provides energyrecovery.

[0044]FIG. 7 illustrates a line driver system made in accordance withanother embodiment of the invention that drives a plurality oftransmission lines of varying length.

[0045]FIG. 8 illustrates another embodiment of the signal generationsystem of the invention.

[0046]FIG. 9 is an alternate embodiment of the line driver system shownin FIG. 6 containing a replenishing system.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0047]FIG. 1 illustrates a prior art line driver system.

[0048] As shown in FIG. 1, a transmission line 1 delivers a voltage VLto a load 3.

[0049] The transmission line 1 is driven by a driver. The driverincludes a signal generation system, including a switch 5, a first drivesignal 7 (which is shown in this example as being ground) and a seconddrive signal 9 (which is shown as being a source of voltage V). Thesignal generation system is connected to a controller 11 whichelectronically controls the switch 5, thus causing V_(IN) at an input 13of the transmission line 1 to switch between the first drive signal 7(ground) and the second drive signal 9 (V). In a typical configuration,the switch 5 is an electronic switch, such as a FET, MOSFET, SCR, triacor BJT.

[0050] The goal of the prior art line driver system shown in FIG. 1, ofcourse, is to cause V_(L) at an output 15 of the transmission line thatis across the load 3 to switch between the levels of the first drivesignal (ground) and the second drive signal (V) in synchronism with thecontrol signal generated by the controller 11. In a typical system, thecontroller 11 receives data as a serial stream of logical “ones” and“zeros.” The net result is that the data is delivered to the load 3.

[0051] In practice, however, the signal that is generated by atransition (rising or falling) in V_(IN) at the input 13 to thetransmission line 1 is reflected at the output 15 of the transmissionline 1 back to the input 13. This occurs when, for instance, theimpedance of the load 3 does not match the impedance of the transmissionline 1.

[0052] When the reflected signal arrives back at the input 13 to thetransmission line 1, it is usually again reflected back to the output 15of the transmission line 1. This typically occurs because the impedanceof the signal generation system is also not the same as the impedance ofthe transmission line 1.

[0053] This now twice-reflected signal is again reflected when itreaches the output 15 of the transmission line 1 back to the input 13.This process repeats until the losses in the system reduce the magnitudeof a reflection to zero. Typically, the magnitude of the reflectedsignal decays exponentially. This phenomena is often referred to as“ringing” because of its oscillatory nature.

[0054]FIG. 2 illustrates the ringing that typically occurs in the priorart line driver system shown in FIG. 1. The top of FIG. 2 illustrates atypical profile 21 for V_(IN). As shown in this profile, V_(IN)typically transition from a first voltage level (0 in this example) to asecond voltage level (V in this example) and then later transitions backto the first voltage level.

[0055] A profile 23 is shown of the voltage V_(L) that is deliveredacross the load. As shown by FIG. 2, however, V_(L) does not alwaysmatch V_(IN). Not only is it slightly delayed in time (due to the timeit takes the signal to travel through the transmission line 1), butthere is noticeable “ringing” following each transition.

[0056] Of course, it is to be understood that the magnitude of theringing, as well as the number of cycles in the ringing, will varywidely, depending upon the parameters of the system.

[0057] As indicated above, this ringing can be quite problematic. Itexposes circuitry connected to both the input 13 and the output 15 ofthe transmission line to higher-than-normal voltage excursions,requiring the voltage ratings on these components to be increased beyondwhat would otherwise be needed. The ringing itself can also bemisinterpreted by the circuitry as constituting a change in the datasignal, creating the possibility of a data error.

[0058]FIG. 3 illustrates a line driver system made in accordance withone embodiment of the invention.

[0059] As in FIG. 1, FIG. 3 includes a transmission line 1 having aninput 13 and an output 15 connected a load 3. Unlike FIG. 1, however,the system shown in FIG. 3 utilizes three drive signals, a first drivesignal 17 (shown as ground), a second drive signal 19 (shown as V/2) anda third drive signal 21 (shown as V). The input 13 to the transmissionline 1 is connected to a switching system 23 that causes the input 13 tothe transmission line 1 to switch between one of the drive signals. Theswitching system 23, in turn, operates under the control of a controller25.

[0060]FIG. 4 illustrates the reduction in the ringing that occurs withthe embodiment of the invention shown in FIG. 3. As shown in FIG. 4, thecontroller 25 causes the switching system 23 to switch the input 13 ofthe transmission line 1 from the first drive signal 17 (shown asground), to the second drive signal 19 (shown as V/2) and then to thethird drive signal 21 (shown as V). Thereafter, the controller 25 causesthe input 13 to the transmission line 1 to be connected back again tothe second drive signal 19 (shown as V/2) and then the first drivesignal 17 (shown as ground). This is illustrated in a profile 31.

[0061] The corresponding output voltage V_(L) is illustrated in aprofile 33 in FIG. 4.

[0062] As shown in FIG. 4, the load V_(L) begins to transition from thefirst voltage level (ground) to the second voltage level (V/2) at timetd after the input voltage V_(IN) makes this transition. The time tdrepresents the time it takes a signal to travel from the input 13 of thetransmission line 1 to the output 15 of the transmission line 1.

[0063] As soon as the transition is received at the output 15 of thetransmission line 1, it is reflected back to the input 13 because of theimpedance mismatch. However, the signal from the input 13 is stilltraveling to the output 15. The reflected signal is therefore added tothe incoming signal, causing V_(L) at the output 15 of the transmissionline 1 to rise all the way up to the level of the third input signal(V), as also shown in FIG. 4.

[0064] The controller 25 is configured to cause the switching system 23to switch the input 13 to the third drive signal 21 (V) at approximatelythe moment the reflected signal first arrives back at the input 13 tothe transmission line 1. The reflected signal is thus met with a newincoming signal, which is approximately equal in magnitude, thussubstantially reducing any further reflections, even though there mightbe an impedance mismatch between the impedance of the transmission lineand the impedance of the drive system. Mathematically, the step from thesecond drive signal level to the third drive signal level is timed to beapproximately twice the signal delay time of the transmission line 1,2t_(d), as also illustrated in FIG. 4.

[0065] The net result is that the ringing is reduced, as alsoillustrated in FIG. 4. Accordingly, the output voltage V_(L) across theload 3 transitions smoothly from the level of the first drive signal(ground) to the level of the third drive signal (V) without noticeableringing.

[0066] The reverse process is preferably followed during the downwardtransition of V_(IN) from the level of the third drive signal (V) to thelevel of the first drive signal (ground), as also shown in FIG. 4. Asshown in FIG. 4, this downward transition is also made in two steps. Thefirst step is a transition from the level of the third drive signal (V)to the level of the second drive signal (V/2). Following a delay ofapproximately 2t_(d) (the time needed for the transition to the level ofthe second drive signal to be reflected from the end 15 of thetransmission line 1 back to the beginning 13 of the transmission line1), the second transition to the level of the first drive signal(ground) is made at the output 15. As shown in FIG. 4, this similarlycauses the output voltage V_(L) across the load 3 to transition smoothlyfrom the level of the third drive signal (V) to the level of the firstdrive signal (ground) without appreciable ringing.

[0067] The voltages of the various drive signals can vary widely.Although the level of the first drive signal 17 is shown as beingground, it is to be understood that the level of the first drive signal17 could, in fact, be a negative or a positive voltage with respect toground. The level of the second drive signal 19 is between the level ofthe first drive signal 17 and the level of the third drive signal 21, asin the example discussed above.

[0068] In one embodiment, the level of the second drive signal 19 ismidway between the level of the first drive signal 17 and the level ofthe third drive signal 21.

[0069] In another embodiment, the magnitude of the reflected signal thatarrives back at the input 13 may be something less than twice themagnitude of the signal that is sent to the output 15. This can happen,for example, when the impedance of the load 3 is somewhat matched to theimpedance of the transmission line 1. In this case, the level of thesecond drive signal is somewhat above the midway level on the risingtransition, so that the level of the third drive signal matches thereflected level of the second drive signal; and the level of the seconddrive signal is somewhat below the midway level on the fallingtransition, so that the level of the first drive signal matches thereflected level of the second drive signal.

[0070] In a still further embodiment, the level of the second drivesignal 19 is midway between the level of the first drive signal 17 andthe level of the third drive signal 21, even when the impedance of theload 3 is somewhat matched to the impedance of the transmission line 1.Although this will not reduce the ringing as much, it is often easier togenerate such a midway level signal.

[0071] The timing of each rising transition from the level of the firstdrive signal 17 to the level of the second drive signal 19 and thetiming of the falling transition from the level of the third drivesignal 21 to the level of the second drive signal 19 are usuallygoverned by the timing of the data stream that is to be sent to the load3 over the transmission line 1.

[0072] On the other hand, the timing of the rising transition from thelevel of the second drive signal 19 to the level of the third drivesignal 21 and the timing of the falling transition from the level of thesecond drive signal 12 to the level of the first drive signal 17 aregoverned by the length of the transmission line 1 and, moreparticularly, by the time it takes a signal to travel round-trip throughthe transmission line 1. This amount is noted on FIG. 4 as 2_(t)d.

[0073] The controller 25 is configured to cause the switching system 23to sequentially switch from the level of the first drive signal 17 tothe level of the second drive signal 19 and then to level of the thirddrive signal 21, and to then sequentially switch from the level of thethird drive signal 21 to the level of the second drive signal 19 andthen to the level of the first drive signal 17 in accordance with thesetiming requirements. As indicated above, the timing of the switchbetween the level of the first drive signal 17 and the level of thesecond drive signal 19 on the rising edge and the switch between thelevel of the third drive signal 21 and the level of the second drivesignal 19 on the falling edge are externally governed by the timing ofthe data stream that is to be delivered. On the other hand, the timingof the rising transition from the level of the second drive signal 19 tothe level of the third drive signal 21 and the falling transition fromthe level of the second drive signal 19 to the level of the first drivesignal 17 are governed by internal considerations, namely the time ittakes the earlier drive signal to make a round trip through thetransmission line 1.

[0074] In one embodiment, this internal timing is established byempirical calculation or by experimental testing of an actualtransmission line. Once determined, the controller 25 is programmed withthis delay information, thus operating in an “open loop” mode.

[0075] In another embodiment, a sensing apparatus (not shown) isconnected to the input 13 of the transmission line 1 to detect when areflection of a transition arrives back at the input 13 to thetransmission line 1. Upon detection, the sensing system (not shown)communicates with the controller 25 to initiate the next transition thatis needed. In this “closed loop” configuration, the second step isinitiated based on the detection of the return of an actual reflection,not based on a prior calculation or measurement.

[0076] Although having thus-far described the input signal V_(IN) asbeing composed of three drive signals each at different levels, it is tobe understood that the input signal V_(IN) could be composed of agreater number of drive signals, such as 5 or 7, as shown in FIGS. 5(a)and (b), respectively. In this instance, of course, an equivalent numberof supplies would be needed, one for each input level. Similarly, theswitching system, such as the switching system 23 shown in FIG. 3, wouldneed to be configured to sequentially switch between each drive signal.Correspondingly, the controller, such as the controller 25 shown in FIG.3, would need to be configured to cause the switching system tosequentially switch between the different drive signals. The timing ofthe first rising step and the first falling step would, again, beexternally governed by the timing of the data signal that is to bedelivered to the load 3. Each of the subsequent even steps (e.g., thesecond or fourth step) would be timed to begin the moment the reflectionof the transition from the immediately preceding step arrives back atthe beginning 13 of the transmission line 1. The timing of the remainingodd-numbered steps (e.g., steps 3 or 5) could vary. Preferably, though,the timing of the remaining odd-numbered steps would be short tomaximize the speed of data transfer. Indeed, each remaining odd-numberedstep could be made at the same time as its preceding even numbered step.In this embodiment, each pair of even and odd numbered steps would bemerged into a single step, thus reducing the total number of steps andsupplies.

[0077] One continuing problem when driving transmission lines with datasignals is energy losses that occur during signal transitions. Theselosses become particularly great when the transmission line 1 has alarge resistance. These energy losses can be particularly problematic inlow-power applications, such as in a VLSI pin driver used for fastchip-to-chip communication.

[0078]FIG. 6 illustrates a line driver system made in accordance withanother embodiment of the invention that, in addition to ring reduction,minimizes these energy losses. The system is the same as the systemshown in FIG. 3, except that the second drive signal 19 is generated bya capacitor 41, instead of a source, such as the V/2 shown in FIG. 3.This modification adds an energy-recovery function to the invention.

[0079] When first energized, the switch from the first drive signal 17(ground in this example) to the second drive signal 41 will have noeffect. V_(IN) will remain at the level of the first drive signal 17(ground). The succeeding switch to the third drive signal 21 (V in thisexample) will then cause V_(IN) to jump to the third drive signal (V),much as in the prior art system shown in FIG. 1. Unless the impedance ofthe load is matched to the impedance of the transmission line 1, theoutput voltage V_(L) will suffer from ringing following this firsttransition, again much like in the system shown in FIG. 1.

[0080] During the first falling transition, the switching system 23switches from the third drive signal 21 (V) to the second drive signal41. Although the level of the second drive signal 41 starts out at thelevel of the first drive signal (ground in this example), the currentthat flows into the capacitor 41 from the load 3 charges it to a levelbetween the first drive signal 17 (ground) and the third drive signal 21(V).

[0081] During the second rising transition, the switch from the firstdrive signal 17 to the second drive signal 41 causes a small transitionin V_(IN). The amount of ringing caused by the second rising transitionis thus reduce somewhat.

[0082] During the second falling transition, the capacitor 41 will startout at a level between the third drive signal 21 and the first drivesignal 17. This will reduce the ringing that is caused by the secondfalling transition somewhat. During the third rising transition, theinitial voltage across the capacitor 41 will be even higher, thusfurther reducing the ringing that is caused by the third risingtransition.

[0083] After several rising and falling transitions, the voltage levelon the capacitor 41 stabilizes to approximately V/2, the level that inmost embodiments maximizes the reduction of ringing during both risingand falling transitions. Thereafter, the system in FIG. 6 functions toreduce ringing, just like the system in FIG. 3.

[0084] Unlike the system in FIG. 3, however, the system in FIG. 6 isaccomplishing another important function—energy conservation. In bothFIG. 3 and FIG. 6, energy is delivered into the transmission line 1 andthe capacitive load 3 during each rising transition and is then removedfrom the capacitive load 3 and the transmission line 1 during eachfalling transition. In FIG. 3, however, most of the removed energy isdissipated as heat in the intrinsic resistances of the switching system23 and the internal impedances of the drive signal supplies. In FIG. 3,on the other hand, a significant portion of that returning energy isstored in the capacitor 41. Instead of being dissipated or otherwisewasted, this stored energy is then reused during the next risingtransition. The system in FIG. 6 thus conserves energy that wouldotherwise have been wasted, while simultaneously reducing ringing, allwithout adding any components to the system that dissipate additionalenergy.

[0085] Although having described this energy-recovery embodiment of theinvention as utilizing a capacitor, i.e., capacitor 41, other forms ofenergy-storage devices could be used instead, such as an inductor, acombination of an inductor or capacitor, or others combinations ofcomponents.

[0086]FIG. 7 illustrates a line driver system made in accordance withanother embodiment of the invention that drives a plurality oftransmission lines of varying length. As shown in FIG. 7, a load 51 isdriven at an output 53 of a transmission line 55 having an input 57; aload 61 is driven at an output 63 of a transmission line 65 having aninput 67; and a load 71 is driven at an output 73 of a transmission line75 having an input 77.

[0087] As also shown in FIG. 7, the lengths of the transmission lines55, 65 and 75 are different, and their inputs 57, 67 and 77 are allconnected to the output of a switching system 81. The inputs to theswitching system 81, such as inputs 83, 85 and 87, are each connected toa drive signal. Although not shown, it is to be understood that theswitching system 81 may have additional inputs connected to other drivesignals. A controller 83 controls switching of the switching system 81to cause it to sequentially switch between the various drive signals. Aswith the systems described in FIGS. 3 and 6, the first rising step andthe first falling step are externally governed by the timing of the datasignal to be delivered. The remaining even steps are timed to coincidewith the arrival of reflections, such that each reflection is met by anequivalent input signal, thus reducing any further reflection that mightotherwise be caused. As each new step introduced by the driver willtravel down all the transmission lines (with the exception of the linewhose incoming transition the new step served to cancel), the number ofreflected transmissions requiring new steps for canceling will growquickly unless the lengths of the transmission lines are related tosmall integers. As each reflected transition will in practice have beendamped somewhat compared to the outgoing transition, there will be apoint of diminishing returns where the power lost through ringing willbe smaller than the power spent in the line driver on generatingadditional steps.

[0088]FIG. 8 illustrates another embodiment of the signal generationsystem of the invention. Thus far, the switching systems, such as theswitching system 23 shown in FIGS. 2 and 6 and the switching system 81shown in FIG. 7, have been illustrated as a single pole, multiple throwswitch. Although such a choice simplifies the discussion that has thusfar been presented, single pole, multiple throw electronic switches arenot common. A more practical approach for implementing these switchingsystem is the approach shown in FIG. 8. As shown in FIG. 8, each drivesignal 91, 93 and 95 is connected to the input 97 of a transmission line(not shown) through its own electronic switch, such as electronicswitches 99, 101 and 103, respectively. Instead of simply controlling asingle switch, a controller 105 is configured to independently controleach of the electronic switches 99, 101 and 103, such that when oneelectronic switch opens, the next in sequence closes. Thus, the use of aplurality of single pole, single throw switches as the switching systemcan readily be adapted to any number of drive signals, such as the fivelevels shown in FIG. 5(a) or the seven levels shown in FIG. 5(b).

[0089] Again, each of these switches are preferably electronic, such asa MOSFET, SCR, triac or BJT. Other configurations known in the art forthe switching system, of course, are also contemplated.

[0090]FIG. 9 is an alternate embodiment of the line driver system shownin FIG. 6 with a replenishment subsystem. All of the components in FIG.9 are the same as those in FIG. 6 with the exception of a switch 117 andan additional connection 119 between the switch 117 and the controller23.

[0091] In operation, the voltage across the capacitor 41 may slowlydecline from the V/2 value because of losses in the transmission line 1.By periodically activating the switch 117 from a control signal from thecontroller 23 over the control line 119, the V/2 value across thecapacitor 41 can be replenished. Preferably, this refreshing would bedone at a moment in time when the capacitor 41 is not connected to theline 1 through the switch 23. Of course, it could also be refreshed whenit is connected.

[0092] Although having thus far described certain embodiments of theinvention, it is to be understood that the invention embraces many otherembodiments and configurations and has many other benefits. Theinvention is limited solely by the claims that now follow.

1. A method of driving a transmission line, wherein a first end of thetransmission line is connected to a driver and a second end of thetransmission is connected to a load, and wherein the impedances of thedriver and the load are different from the impedance of the transmissionline, the method comprising: operating the driver so as to apply aplurality of drive signals to the transmission line such that ringing onthe transmission line is reduced.
 2. A driver for driving a transmissionline, wherein a first end of the transmission line is connected to saiddriver and a second end of the transmission is connected to a load, andwherein the impedances of the driver and load are different from theimpedance of the transmission line, the driver comprising a signalgeneration system for operating the driver so as to apply a plurality ofdrive signals to the transmission line such that ringing on thetransmission line is reduced.